The present invention generally relates to a developer for use in an electrophotographic machine for developing an electrostatic latent image into a visible image and, more particularly, to an electrographic developer comprising a mixture of a carrier with a toner of the type having a property of being triboelectrically charged to a positive polarity when subjected to frictional contact with the carrier.
The electrophotographic developers now being produced are generally available in two types, a mono-component type and a two-component type. The two-component type developer is more widely used than the mono-component type and includes one which comprises a mass of carrier particles of relatively large particle size (within the range of 80 to 200 .mu.m), for example, iron powder or glass beads, and a mass of fine particles, i.e., so-called "toner" particles essentially consisting of a binder and dispersed in the binder, and one wherein the carrier particles essentially consist of a binder and a powder of magnetizable material dispersed in the binder and have an average particle size within the range of 5 to 30 .mu.m, the second-mentioned two-comonent type developer being an improved version of the first-mentioned developer, which has been developed for the purpose of substantially eliminating some problems inherent in the latter.
In the conventional two-component type developer when the carrier and toner particles are mixed together, they are triboelectrically charged to opposite polarities, the amount of triboelectrical charge on each of the carrier and toner particles being determined by the difference in level between triboelectrical charging series. Although the larger difference in level results in the increased amount of triboelectrical charge, any triboelectrical charge will be no longer generated when the difference in level is made extremely small or when the level of the triboelectrical charging series of the carrier particles and that of the toner particles are made identical with each other.
Adjustment of the amount of triboelectrical charge can in theory be accomplished by suitably selecting the material of either the carrier or the toner relative to the other. However, in practice, since the material of the carrier utilizable in a particular application is determined by the type of developing system, i.e., a cascade developing system or a magnetic brush developing system or any other developing system, the adjustment of the amount of triboelectrical charge is generally carried out by selecting the material of the toner to be mixed with the carrier. The amount of triboelectrical charge generated in the toner is determined mainly by the material for either the carrier or toner. That is to say, it is generally considered that the amount of triboelectrical charge on the toner is high for a large distance between the position of the toner material on the triboelectrical charging series and the position of the carrier material on the triboelectrical charging series. In view of this, in order to realize a difference between the position of the toner material on the triboelectrical charging series and that of the carrier material on the triboelectrical charging series, it has been a practice in the art to add a charge controller to the toner, to adjust the content of carbon black which also serves as a pigment, to change the kind of a resin and/or the kind of a monomer of a copolymeric resin, and/or to blend different kind of resins.
With respect to the polarity of triboelectrical charge, the toner can be imparted with a property of being charged to a positive polarity when an electron donor substance operable to expel electrons is added thereto, or with a property of being charged to a negative polarity when an electron acceptance substance operable to attract electrons is added thereto. The polarity of triboelectrical charge on the toner is affected also by the kind of the resin used, and it has long been well known that a styrene resin tends to exhibit the property of being charged to a negative polarity and that a resin containing nitrogen in its molecule tends to exhibit the property of being charged to a positive polarity.
Bearing the above in mind, where the electron donor substance is added to the toner of a composition containing a styrene resin so that the toner can be imparted with the property of being triboelectrically charged to a positive polarity, the triboelectrical property of the toner lacks stability and the resultant toner tends to fail to retain the initially charged positive potential for a prolonged period of time since the triboelectrical property of the electron donor substance is in opposition to that of the styrene resin. In addition, there is another case in which the toner tends to fail to retain the initially charged potential for a prolonged period of time. By way of example, when a developer which is prepared by mixing the toner with the carrier and then the toner is triboelectrified by frictional contact with the carrier and is allowed to stand for a prolonged period of time, the potential initially charged on the toner during the preparation of the developer attenuates or decays exponentially, although the rate of attenuation or decay varies depending on the conditions under which the developer is stored or shelved. The higher the temperature and/or humidity, the faster the potential decay. Conversely, the lower the temperature and/or humidity, the slower the potential decay.
However, in practice, the problem associated with the decay of the initially charged potential can be negligible except for the case with the developer of a type wherein the potential initially charged on the toner tends to attenuate extraordinarily rapidly. This is because the developer, when in use in an electrophotographic copying machine, is stirred within a developer tank or hopper and, therefore, even though the potential on the toner decays to a certain extent (the maximum possible half-life (.UPSILON. value) of the potential on the toner used in the currently commercially available developer mix of this kind is about one week.) while the machine is not in operation, the toner can again be charged in frictional contact with the carrier during the stirring that takes place within the developer tank or hopper. However, not only because the developer is in most cases exposed to numerous environmental conditions during a period subsequent to the preparation thereof until it is charged into the machine or it is replaced with the one being used, but also because it will take a relatively long period of time subsequent to the preparation thereof and before it is actually used in the machine, there are some problems associated with the decay of the initially charged potential on the toner, which problems cannot be resolved merely by stirring the developer within the developer tank or hopper during the developing process being performed by the machine.
By way of example, where the development of the electrostatic latent image into a visible image is carried out by the use of the developer containing the toner, the initially charged potential of which has decayed during the storage of such developer, the use of such developer brings about some adverse influences on the resultant visible image, for example, the excessively high density, the undesirably foggy background of the developed image, the undesirable scattering of toner particles which would take place during transportation of particles of the developer on a developing sleeve in the machine.
In view of the above, it has long been desired to manufacture a toner such that the potential initially charged thereon can be retained for a prolonged period of time as occurs before the shipment from its manufacturer and with the minimized attenuation thereof, even when exposed to such numerous environmental conditions as they may occur.